A blood circuit, which is used for dialysis treatment so as to extracorporeally circulate blood of a patient, is generally configured to mainly include a flexible tube for connecting respective configuration elements to each other, such as a dialyzer and a drip chamber. A co-injection member (also called a “rubber button” or an “access port”) is connected to a predetermined position of the blood circuit. The co-injection member is normally configured to have a main body that can be connected to the blood circuit, an internal flow route through which a liquid of the blood circuit can be circulated, an inlet flow route that introduces the liquid into the internal flow route, an outlet flow route that discharges the liquid of the internal flow route, and a rubber member that is attached to an opening formed in the main body.
Then, the blood flowing in the blood circuit can be collected, or a drug or a physiological saline solution can be injected into the blood circuit by a puncture needle of an injection syringe puncturing the rubber member of the co-injection member. Some rubber members of the co-injection member have a slit formed for communication between the inside and the outside of the internal flow route. The blood flowing in the blood circuit can be collected, or the drug or the physiological saline solution can be injected into the blood circuit by inserting a distal end of a puncture tube or a syringe into the slit. However, there is a possibility that flow of the blood may congest the internal flow route. Accordingly, in order to prevent the congestion, in the related art, a co-injection member (refer to PTL 1) has been proposed in which a projection is formed on a bottom surface in a central portion of the internal flow route, or a co-injection member (refer to PTL 2) has been proposed in which the inlet flow route and the outlet flow route are formed toward a position shifted from the center of the internal flow route.